Radio shielded magneto



July 24, 1956 R. E. LEAR ET AL RADIO SHIELDED MAGNET Filed Nov. 18, 1954INVENTOR5 R. E. LEAR BY B.A. GLASSMACH ER United States Patent RADIOSHIELDED MAGNETO Robert E. Lear and Bernard A. Glassmacher, Columbus,Ohio, assignors to Surface Combustion Corporation, Toledo, Ohio, acorporation of Ohio Application November 18, 1954, Serial No. 469,675

8 Claims. (Cl. 310153) This invention relates to a radio shieldedmagneto, and more particularly, to such a magneto having substantiallyreduced size and weight as compared with previously known radio shieldedmagnetos.

In many aircraft installations, for example, the use of magnetos isadvantageous. As a specific example, magnetos are used in ground heatersto generate an E. M. F. for providing an igniting spark. Such groundheaters are frequently used at military air bases, as well as atcommercial air fields. The use of previously known magnetos in suchheaters has been impossible because of radio interference unless theentire magneto was enclosed in a suitable shield which blocked the airtransmission of energy from the magneto. The presence of such a shieldaround a magneto greatly increases the difiiculty of servicing, and addssubstantial bulk and weight to the unit. Particularly in the case ofportable ground heaters, which must sometimes be carried by air, forexample to advance bases, the extra bulk and weight of the shielding arealso objectionable.

The instant invention is based upon the discovery of a radio shieldedmagneto having virtually the same bulk and weight of a correspondingunshielded magneto, and one where there is no problem of servicingbecause connections are made in the usual manner directly to themagneto, rather than through a surrounding sheath or shield.

It is, therefore, an object of the invention to provide an improvedradio shielded magneto.

Other objects and advantages of the invention will be apparent from thedescription which follows, reference being had to the accompanyingdrawings, in which- Fig. l is a plan view of the stator of a magnetoaccording to the invention, showing various electrical parts attachedthereto;

Fig. 2 is a vertical sectional view generally along the line 2-2 in Fig.1, showing the assembled magneto; and

Fig. 3 is a view in vertical elevation of the exterior of the stator ofthe magneto.

Referring now more particularly to the drawingsythe specific radioshielded magneto according to the invention shown in Figs. 1 through 3comprises a flywheel or rotor 11, and a stator 12 carrying variouselectrical apparatus conventional in magnetos, as well as part of theshielding mechanism according to the invention.

The main essential electrical apparatus carried by the stator comprisesa pair of coils 13 wound on separate arms of a core 14 attached to thestator by machine screws 15, a grounded contact 16, a second contact 17urged toward the contact 16 by a spring 18 electrically connectedthereto, and supported on an insulating base 19 pivotable around a stud20. A wire 21 connects one of the coils 13 to the spring 18; a wire 22connects a condenser 23 to the spring 18; and a shielded wire 24connects an operating switch member (not illustrated) to the spring 18.The Wires 21, 22 and 24 are retained in position by a machine screw 25.An E. M. F. is generated in the coils 13 from which it is availablethrough wires 26, for ex- 2,756,355 Patented July 24, 1956 ample forproviding a current to energize igniter plugs, by virtue of rotation ofthe rotor 11 which carries permanent magnets 27. The portions of themagneto of Figs. 1 through 3 thus far discussed, and the operation, areconventional, and constitute no part of the instant invention.

The radio shielding of the magneto of Figs. 1 through 3 comprises, inthe specific embodiment of the invention shown, a knitted Monel gasket30 retained by a circular angle member 31 mounted on the stator 12 bymachine screws 32, in electric and mechanical contact with an outersurface 33 of the stator. As will be seen by reference to Fig. 2, theknitted Monel gasket 30, when the magneto is in operation, makes rubbingmechanical and electrical contact with the rotor 11. It has been foundthat merely the provision of a knitted Monel gasket 30 positioned andsupported as shown in Fig. 2, in mechanical and electrical contact withthe stator and in rubbing mechanical and electrical contact with therotor, accomplishes substantial radio interference reduction in thefrequency range from 0.15 megacycle to megacycles. The knitted Monelgasket has a service life in excess of 250 hours of continuousoperation, and is, therefore, within 1954 specifications set by theUnited States Air Corps. It has also been found that other gasketmaterials can be substituted for the knitted Monel gasket 30, withsimilar effect upon radio interference suppression. For example, a solidcopper gasket has been substituted for the Monel gasket 30 and foundsatisfactory from the standpoint of radio interference suppression inthe range of frequencies from 0.15 megacycle to 150 megacycles. Thecopper gasket, however, is commercially impractical because it lacksabrasion resistance, and has, therefore, a short useful life.

In a preferred embodiment of the invention, as will be seen by referenceto Fig. 2, at least the inner surface 34 of the rotor 11 has acomparatively high electrical conductivity, for example by virtue of acadmium plating 35 provided at least on such surface. In the case of amagneto having a stamped steel rotor, although the provision of theMonel gasket 30, as described, effected substantial suppression of radiointerference, the interference level of the magneto provided with theMonel gasket was still above 1954 U. S. Air Corps specifications.However, the provision of cadmium plating 35 on the surface 34 of therotor reduced the level of radio interference to a point within suchspecifications over the entire range indicated. In addition to reducingradio interference, cadmium plating as described protects a ferrousmetal rotor against corrosion in service conditions, and thus avoids thenecessity for one of the usual protective coatings, which have beenfound to increase radio interference.

Although the invention is not limited to the following theoreticalexplanation, it is believed that a radio shielded magneto according tothe invention is operative because electrostatic charges generatedduring operation of the magneto are interrupted by the conductivecadmium plate 35, conducted along the surface thereof, through the Monelgasket to the stator 12, and ground ed through the metal portion of theapparatus carrying the magneto, for example a gasoline engine, to whichthe stator is attached. In this connection, it has been observed that acommercially available magneto having a stamped steel rotor and providedwith a knitted Monel gasket, as described, emitted more radiointerference signals than permitted by 1954 U. S. Air Corpsspecifications in the frequency range previously mentioned when thestamped steel rotor supplied by the manufacturer was used, and also whenthis rotor was plated with copper by ordinary commercial platingtechniques. When, however, a like magneto was used for test purposes,except that the rotor was plated with cadmium by ordinary commercialtechniques, the emission of radio interference signals was reduced to apoint within the said specifications. The electrical conductivity ofcadmium is approximately one-fourth that of copper, but it is known thatthe deposition of a continuous copper plate is considerably morediflicult than is the deposition of a continuous cadmium plate. It isalso known that electrical energy, in the frequency range involved, isconducted as a surface phenomenon, and will develop, within practicallimits, whatever voltage is necessary to cause its conduction. it isbelieved, therefore, that the superiority of cadmium plating, overcopper plating or an unplated magneto rotor, is attributable to theincreased continuity, as regards electrical conductivity, achieved bycadmium plating of at least the interior surface of the rotor. It hasalso been demonstrated that cadmium plating on only the interior of therotor is equally as effective as cadmium plating on both the interiorand exterior thereof. It is preferred that at least the interior surfaceof the rotor have a resistivity not greater than 20 microhms percentimeter cube. It will be apparent that, for radio shielding accordingto the invention to be effective, the rotor, the stator, and the gasket30 or its equivalent must form an enclosure for the various electriccomponents of the magneto.

The ideal radio shielded magneto according to the invention is onewherein a continuous, high electrical conductivity path is provided fromthe interior surface of the rotor, and through the stator to ground. Asa practical matter, however, it is preferred to sacrifice the highconductivity of copper plating, for example, on the interior of therotor, because of the greater ease with which continuity is achieved bycadmium plating, for example. It is also preferred to sacrifice the highelectrical conductivity of a copper gasket making connection between therotor and the stator in favor of the greater abrasion resistance ofMonel, for example.

It will be apparent that various changes and modifications can be madefrom the specific details disclosed and discussed without departing fromthe spirit Of the attached claims.

What we claim is:

1. In a radio shielded device for generating electricity, comprising anarmature carried by a stator, at least one magnet carried by a flywheel,and wherein the stator and the flywheel are relatively movable and forman enclosure for the armature and the magnet except for an annular spaceseparating the stator and the flywheel,

the improvement which includes a resilient, knitted,

Monel gasket interposed in and substantially filling the annular space,attached to and in electric contact with the stator and in rubbingelectric contact with the flywheel, and a substantially continuous layerof cadmium plate constituting at least the interior surface of theflywheel.

2. In a radio shielded device for generating electricity, comprising anarmature carried by a stator, at least one magnet carried by a flywheeland wherein the stator and the flywheel are relatively movable and forman enclosure for the armature and the magnet except for an annular spaceseparating the stator and the flywheel, the improvement which includes aresilient, knitted, wearresistant metal gasket interposed in andsubstantially filling the annular space, attached to and in electriccontact with the stator and in rubbing electric contact with theflywheel, and a substantially continuous layer of a metal having anelectric resistivity not greater than 20 microhms per centimeter cubeconstituting at least theinterior surface of the flywheel.

3. In a radio shielded device for generating electricity, comprising anarmature carried by a stator, at least one magnet carried by a flywheel,and wherein the stator and the flywheel are relatively movable and forman enclosure for the armature and the magnet except for an annular spaceseparating the stator and the flywheel, the improvement which includes aresilient knitted metal gasket interposed in and substantially fillingthe annular space, attached to and in electric contact with the statorand in rubbing electric contact with the flywheel.

4. In a radio shielded device for generating electricity, comprising anarmature carried by a stator, at least one magnet carried by a flywheel,and wherein the stator and the flywheel are relatively movable and forman enclosure for the armature and the magnet except for an annular spaceseparating the stator and the flywheel, the improvement which includes aresilient, knitted, wearresistant metal gasket interposed in andsubstantially filling the annular space, attached to and in electriccontact with the stator and in rubbing electric contact with theflywheel.

5. In a radio shielded device for generating electricity, comprising anarmature, first plate means for supporting the armature, means forcreating a magnetic field, second plate means for supporting saidlast-named means, and wherein the first plate means and the second platemeans are relatively movable and form an enclosure for the armature andthe means for creating a magnetic field except for an annular spaceseparating said plate means, the improvement which includes a resilientmetal gasket interposed in and substantially filling the annular space,and in electrical contact with both of the plate means.

6. In a radio shielded device for generating electricity, comprising anarmature, first plate means for supporting the armature, means forcreating a magnetic field, second plate means for supporting saidlast-named means, and wherein the first plate means and the second platemeans are relatively movable and form an enclosure for the armature andthe means for creating a magnetic field except for an annular spaceseparating said plate means. the improvement which includes a resilientknitted metal gasket interposed in and substantially filling the annularspace, attached to, and in electric contact with one of the plates andin rubbing electric contact with the other of the plates.

7. In a radio shielded device for generating electricity, comprising anarmature, first plate means for supporting the armature, means forcreating a magnetic field, second plate means for supporting saidlast-named means, and wherein the first plate means and the second platemeans are relatively movable and form an enclosure for the armature andthe means for creating a magnetic field except for an annular spaceseparating said plate means, the improvement which includes asubstantially continuous layer of a metal having an electric resistivitynot greater than 20 microhms per centimeter cube constituting at leastthe interior surface of the movable plate, and means electricallyinterconnecting the two plate means.

8. In a radio shielded device for generating electricity, comprising anarmature, first plate means for supporting the armature, means forcreating a magnetic field, second plate means for supporting saidlast-named means, and wherein the first plate means and the second platemeans are relatively movable and form an enclosure for the armature andthe means for creating a magnetic field except for an annular spaceseparating said plate means, the improvement which includes, aresilient, knitted, wear resistant metal gasket interposed in andsubstantially filling the annular space, attached to and in electriccontact with one of the plates and in rubbing electric contact with theother of the plates.

References Cited in the file of this patent UNITED STATES PATENTS1,797,545 Churher Mar. 24, 1931 1,896,264 Weichsel Feb. 7, 19332,690,514 Phelan Sept. 28, 19-54

